1. Field of the Invention
This invention relates to a method for reducing sheeting during polymerization of alpha olefins and more particularly to a method for reducing sheeting during polymerization of polyethylene.
2. Summary of the Prior Art
Conventional low density polyethylene has been historically polymerized in heavy walled autoclaves or tubular reactors at pressures as high as 50,000 psi and temperatures up to 300.degree. C. or higher. The molecular structure of high pressure, low density polyethylene (HP-LDPE) is highly complex. The permutations in the arrangement of their simple building blocks are essentially infinite. HP-LDPE's are characterized by an intricate long chain branched molecular architecture. These long chain branches have a dramatic effect on the melt rheology of these resins. HP-LDPE's also possess a spectrum of short chain branches, generally 1 to 6 carbon atoms in length. These short chain branches disrupt crystal formation and depress resin density.
More recently, technology has been provided whereby low density polyethylene can be produced by fluidized bed techniques at low pressures and temperatures by copolymerizing ethylene with various alpha-olefins. These low pressure LDPE (LP-LDPE) resins generally possess little, if any, long chain branching and are sometimes referred to as linear LDPE resins. They are short chain branched with branch length and frequency controlled by the type and amount of comonomer used during polymerization.
As is well known to those skilled in the art, low pressure, high or low density polyethylenes can now be conventionally provided by a fluidized bed process utilizing several families of catalysts to produce a full range of low density and high density products. The appropriate selection of catalysts to be utilized depends in part upon the type of end product desired, i.e., high density, low density, extrusion grade, film grade resins and other criteria.
The various types of catalysts that can be employed to produce polyethylene in fluid bed reactors are generally disclosed in U.S. Pat. Nos. 4,855,370; 4,803,251; 4,792,592; 4,532,311; and 4,876,320.
Also disclosed in said patents is the incidence of "sheeting" in the reaction system when certain of the catalysts are utilized.
A strong correlation exists between sheeting and the presence of excess negative or positive static charges. This is evidenced by sudden changes in static levels followed closely by deviation in temperatures at the reactor wall whereby catalyst and resin particles adhere to the reactor walls due to static forces. If allowed to reside long enough under a reactive environment, excess temperatures can result in particle fusion. These temperature deviations are either high or low. Low temperatures indicate particle adhesion causing an insulating effect from the bed temperature. High deviations indicate reaction taking place in zones of limited heat transfer. Following this, disruption in fluidization patterns is generally evident, catalyst feed interruption can occur, product discharge system pluggage results, and thin fused agglomerates (sheets, regardless whether they come loose from reactor walls) are noticed in the granular product. The critical static voltage level for sheet formation is a complex function of resin sintering temperature, operating temperature, drag forces in the fluid bed, catalyst activation energy, resin particle size distribution and recycle gas composition.
Numerous causes for static charge exist. Among them are generation due to frictional electrification (triboelectrification) of dissimilar materials, limited static dissipation, introduction to the process of minute quantities of prostatic agents, excessive catalyst activities, etc.
It is generally believed that when the charge on the particles reaches the level where the electrostatic forces trying to hold the charged particle near the reactor wall exceed the drag forces in the bed trying to move the particle away from the wall, a layer of catalyst-containing polymerizing resin particles forms a non-fluidized layer near the reactor wall. Heat removal from this layer is not sufficient to remove the heat of polymerization because the non-fluidized layer near the wall has less contact with the fluidizing gas than do particles in the fluidized portion of the bed. The heat of polymerization increases the temperature of the non-fluidized layer near the reactor wall until the particles melt and fuse. At this point other particles from the fluidized bed will stick to the fused layer and it will grow in size until it comes loose from the reactor wall. The separation of a dielectric from a conductor (the sheet from the reactor wall) is known to generate additional static electricity thus accelerating subsequent sheet formation.
The above patents also disclose the methods and techniques for substantially reducing the incidence of sheeting in the reaction system. Thus, U.S. Pat. No. 4,876,320 discloses a method for polymerization of one or more alpha-olefins in a fluidized bed reactor in the presence of a catalyst prone to cause sheeting wherein the static electric charges in the reactor at the site of possible sheet formation is maintained below static voltage levels which could otherwise cause sheet formation.
U.S. Pat. No. 4,792,592 maintains static electric charges in the reactor below sheeting levels by creating areas of localized field strength within the reactor for the promotion of electrical discharge to ground.
U.S. Pat. No. 4,803,251 utilizes a chemical additive which generates either a positive or negative charge responsive to particular static levels in the reactor.
U.S. Pat. No. 4,532,311 teaches the introduction of a chromium containing compound into the reactor in such a manner as to contact the surfaces of the reactor in order to reduce the incidence of sheeting.
The present invention provides an alternate and preferred method for reducing the incidence of sheeting during the fluidized bed polymerization of alpha-olefins which employ catalysts prone to cause sheeting.